The Guild leader faction change service (called "Guild Master Faction Change" by Blizzard) is part of Guild Master Services under their overall Guild Services support (although they are basically the same thing). It allows a guild leader to change the faction of the guild and the guild leader character at the same time for a one time fee of US$40 or €35 (£30). A guild name change is free with this service. Combining this service with a guild leader realm transfer gives a discount of US$10 (faction change US$40 + transfer US$35 - US$10 = US$65 total) or €10 (faction change €35 + transfer €30 - €10 = €55 total; £5 discount, faction change £30 + transfer £25 - £5 = £50 total).
New faction guild Edit
When the guild leader changes faction, the framework of the guild goes with the guild leader as well, including the guild vault, level, perks, and achievements.
Former/original guild Edit
The original guild will continue to exist, but it's guild level will be reset to 1 and will not retain its guild bank or any of its achievements, reputation or progression. The guild leader initiating the faction change will need to choose a new guild leader for the original guild. Various requirements and restrictions apply to the guild leader and the appointee to lead the original guild.